I always bought a second battery with my previous laptops, so I decided to look at getting another one for my new MB.
I was surprised to see that the replacement battery being sold on Apple's Web site is rated at 60 watt-hours, compared to only 45 watt-hours for the one that ships in the MB. The weight isn't shown, but it must be heavier than the stock battery, since it can't be bigger.
Still, this might make enough of a difference to obviate the need for carrying two batteries, assuming that the spec is correct - the Web page says a second battery can double your battery life, which doesn't make sense if it's really higher capacity.
I was surprised to see that the replacement battery being sold on Apple's Web site is rated at 60 watt-hours, compared to only 45 watt-hours for the one that ships in the MB. The weight isn't shown, but it must be heavier than the stock battery, since it can't be bigger.
Still, this might make enough of a difference to obviate the need for carrying two batteries, assuming that the spec is correct - the Web page says a second battery can double your battery life, which doesn't make sense if it's really higher capacity.